1. Field of the Invention
The present invention relates to a process for cleaning a harmful gas. More particularly, it pertains to a process for cleaning a halogenide-based harmful gas such as dichlorosilane, hydrogen chloride, tungsten hexafluoride and chlorine trifluoride that are used in the semiconductor manufacturing industry.
There has been a steady rise in the kinds and usage of gaseous halogenide such as hydrogen chloride, dichlorosilane, chlorine trifluoride and tungsten hexafluoride in recent years as the semiconductor industry and the optoelectronics industry continue to develop. The above-mentioned gases are employed for the formation of crystalline silicon, amorphous silicon or silicon oxide films in the production industries of silicon semiconductors and chemical compound semiconductors and are indispensable substances as etching gases. However, since each of the aforesaid gases exerts a harmful influence on human bodies and the environment on account of their high toxicity, they need to be removed when contained in a gas used in the semiconductor production industry, prior to the discharge in the atmosphere, after being used in the industry.
In addition, hydrolyzable or comparatively less toxic gases such as carbon tetrafluoride, perfluoropropane and sulfur hexafluoride are also employed for the dry etching of silicon films and silicon oxide films in a semiconductor production process, but in the gas discharged through the etching process there is formed a harmful component such as silicon tetrafluoride and fluorine by the reaction between the above-mentioned gases and the film component or by the decomposition of the gases. Such being the case, sufficient care should be taken in discharging a gas from the process.
The aforementioned gases are usually available on the market in the state that the gas is filled in a 0.1 to 47 liters bomb as a pure gas or a diluted gas with nitrogen or the like as exemplified hereunder.
Examples of commercially available gases filled in bombs are as follows:
______________________________________ Gas (chemical Bomb capacity Concentration Weight of filled gas formula) (liter) (%) (kg) ______________________________________ HCl 47 100 25 SiH.sub.2 Cl.sub.2 10 100 10 ______________________________________
In order to prevent the atmospheric air from being directly polluted with a gas in case of its leakage from a bomb, the bomb is used in the state that it is connected to a gas supply piping connected to a semiconductor process while it is housed in a bomb accomodation vessel, so called a bomb box that is connected to a ventilation duct. However, even if a bomb is housed in a bomb box, complete prevention of such dangerous accident that is not secured when the bomb is emptied within only about 5 to 10 minutes by a sudden leakage of the gas therefrom. Under such circumstances, there is eagerly desired a complete and thorough countermeasure capable of sufficiently coping with such accident.
2. Description of the Related Art
As the means for removing gaseous halogenide such as hydrogen chloride, dichlorosilane and boron trifluoride that is contained is a process gas, there have heretofore been used two processes, one being a wet process in which the gaseous halogenide is absorbed and decomposed in a scrubber and the other being a dry process wherein the harmful gas is cleaned by being passed through a packed column which is packed with a cleaning agent comprising an alkaline component impregnated into a porous carrier such as activated carbon, a cleaning agent comprising soda lime as the effective ingredient or a like agent.
However, the above-mentioned wet process generally suffers the disadvantages that it finds difficulty in post-treatment and further that it requires a considerable expense in the maintenance of the equipment used therefor.
On the other hand, the aforestated dry process involves such problems that there are not necessarily obtained a sufficient removal rate and a sufficient removal capacity of the harmful gas with a cleaning agent comprising sodium hydroxide, potassium hydroxide or the like impregnated into a porous carrier such as activated carbon or a cleaning agent comprising soda lime and that the dry process fails to promptly cope with a highly concentrated harmful gas and an emergency of sudden or violent leakage of such gas. In addition, a cleaning agent supported on activated carbon sometimes forms inflammable substances with an extremely reactive gas such as fluorine, involving the hazard of fire. On the contrary, soda lime is liable to cause plugging in a cleaning column due to its deliquescent property in combination with the moisture contained in the gas to be treated and to form remarkably deliquescent calcium chloride when used for passing gaseous chloride such as hydrogen chloride and dichlorosilane. As the result, it becomes a problem that the use of soda lime limits the kinds of gases suitable to be treated.
Such being the case, there has long been desired the materilization of a process for cleaning a harmful gas, especially that discharged from semiconductor manufacturing industry. There has also been desired a process being high in treatment rate and treatment capacity for harmful gases, excellent in removal performance, not only in an emergency when a relatively less concentrated, but a large amount of a harmful gas is leaked from a gas bomb because of its abnormality, but also in the ordinary case where a concentrated harmful gas is discharged from a semiconductor manufacturing industry, and free from the possibility of causing a hazard fire in the case of gas cleaning or the plugging in a cleaning column due to deliquesence of a cleaning agent.
In view of the aforestated situation, intensive research and investigation were concentrated by the present inventors on the solution of the above-described problems. As a result it has been found by them that the use of a cleaning agent comprising zinc oxide, aluminum oxide and an alkali compound can remove a harmful gas in various states with extremely high efficiency and utmost safety. The present invention has been accomplished on the basis of the above-mentioned finding and information.